Minecraft Launcher failed to start, reporting that it had a fatal error whilst trying to start up.

Put the summary of the bug you're having here

What I expected to happen was...:
I expected to be able to test the new snapshot

What actually happened was...:
The Launcher seemed to freeze, and reported a fatal error

Steps to Reproduce:
1. Select the latest snapshot
2. Click Download and Play
3. Successfully downloads, but fails to start